1. Field of the Invention
The present invention relates to a zoom lens composed of three groups and, in particular, to a three-group zoom lens used in cameras for ID photographs and the like.
2. Description of the Prior Art
There have conventionally been known three-group zoom lenses which have a higher degree of freedom in aberration correction as compared with two-group zoom lenses, whereby various kinds of aberration such as spherical aberration and chromatic aberration can be made favorable.
There has been known, for example, a three-group zoom lens such as that disclosed in Japanese Patent Publication No. 5-69209 comprising, successively from the object side, a negative first lens group, a positive second lens group, and a negative third lens group, in which the first and second lens groups are moved at the time of varying power, so as to achieve a so-called negative lead type three-group configuration, thereby attaining a wide angle in terms of photographing field angle, while favorably correcting aberration.
In the prior art disclosed in the above-mentioned publication, however, since a stop is disposed within the second lens group, this stop moves together with the other lenses in the second lens group at the time of varying power. Accordingly, F.sub.NO fluctuates at the time of varying power unless the diameter of the stop is changed. In general, it is unfavorable for F.sub.NO to change greatly at the time of variable power. In particular, in instant cameras for ID photographs or the like, since latitude is narrow therein, it is preferable that change in F.sub.NO at the time of varying power be suppressed as much as possible.